Balanced cell proliferation and death is crucial for homeostasis in mammals. Excessive or insufficient proliferation or cell death could lead to cancerous conditions, autoimmunity, immunodeficiency, and neurodegenerative diseases. Several closely related receptors including tumor necrosis factor receptor I, Fas, and death receptors (DR) 3 and 4/5 can trigger apoptotic death upon ligand engagement. Ablation of Fas mediated apoptosis results in lymphoproliferation disease and autoimmunity. Ligand of DR4/5 was found to kill only tumor but not normal cells. Apoptosis signaling of all four receptors appears to be transduced by the adaptor FADD protein and the protease Caspase 8. Surprisingly, FADD-deficiency not only abrogates apoptosis but also causes defective proliferation in T lymphocytes, and probably in other cell types, which leads to early embryonic lethality in mouse. This indicates that FADD may have additional roles in alternative signaling pathways, not necessarily related to cell death. Although there is a reasonable understanding of the mechanism of FADD-mediated apoptosis, it is not clear how FADD regulates cell proliferation required for embryogenesis and lymphocytes development and proliferation. Further dissection of multi-functions of FADD will help reveal novel pathways, which may be targets for therapeutic intervention of many diseases. In this proposed study, recently developed inducible gene targeting and novel transgenetic approaches will be employed to further analyze the physiological function of FADD in mice, in order to help map the signaling network involving FADD. Specifically our objectives are: (1) Using the FADD-deficient+RAG-1-deficient double mutant chimeric mouse model to analyze the T cell proliferation defects. Tissue-specific and inducible FADD-deficient mouse models will be developed to analyze the temporal requirement of FADD function during embryogenesis, and lymphocyte development and proliferation. (2) To dissect multi-functions of FADD by biochemical and transgenic approaches, involving mutational analysis of FADD in cell lines and in mouse. (3) To investigate the role of FLIP and Caspase 8 in FADD-mediated proliferation by in vitro and in vivo mutational studies.